Since there is so much uncertainty as to the accuracy of both the
Metrostyle markings and the marked Tempo as well, it does not seem
like a high priority to accurately calibrate the Tempo indicator.
I would tend to just use both indications as relative measures instead
of an absolute indications.  Having said that, it should be possible to
calibrate the indicator over the whole scale for an individual piano
if you really want to.

First it makes sense to get everything right that you can as everything
that effects the wind motor is going to influence the calibration so
once you calibrate it, you will mess it up again if you fix something
that was wrong.  So start by making sure that there are no leaks, the
valves are timed correctly and that there excess friction in any of the
bearings.  Be sure that the wind motor pressure regulator is working
correctly.

Tempo is a linear scale of feet per minute and the rotation of the
wind motor is also nearly linear as you vary the flow at a fixed
vacuum.  There are going to be slight variations from linear as
friction in bearings and resistance to flow are not strictly linear
but these are all minor concerns and can be eliminated in a calibration
routine if you are willing to do what is really needed.

You can start with the assumption (not necessarily true) that the
original manufacture was perfect and that the Tempo slider valve was
made correctly.  If this is the case then setting the wind motor
governor to the suggested vacuum and getting everything else right
should result in perfect Tempo.  This is not likely to be the case for
a variety of reasons but it can be compensated for if you are willing
to modify the Tempo port itself.

As long as the wind motor governor is providing a fixed vacuum level,
you can get any Tempo you want by varying the flow through an orifice.
The Tempo port and its slider provides this capability.  It is usually
shaped like a bugle or a simple wedge.  You can vary the shape to get
exactly the Tempo you want at any slider position as long as you want
a continuous progression from end to end.

If you do not want to spend hours timing the run of a test roll, you
can get a good idea of Tempo with a tachometer that has a surface speed
wheel.  You can run the Tempo indicator up and down and note the places
where it is not correct and modify the port to achieve the needed
results.  You will probably need to make several port plates before you
get it right.

I would not modify the original and would try your attempts in plastic
until you get it right.  As fun as this might be, it is probably a
total waste of time for the above mentioned reasons.

Best regards,
Spencer Chase
Garberville, California
http://www.spencerserolls.com/